Term
| Briefly describe the basic chain of events leading to the formation of a star like the Sun? |
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Definition
| The sun must have started out as a large collapsing cloud of gas inside some ancient interstellar cloud. The cloud collapsed for millions of years until it formed a rotating disk with a large central bulge. The central bulge continued to collapse under its own gravity until it gave off thermonuclear energy. |
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Term
| What is the role of heat in the process of stellar birth? |
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Definition
| You have to have a concentration of heat in order to reach the threshold of fusion. Gravitational compression is the other essential element. That's the star's ignition phase we're talking about. The internal temperature of a star has to reach 10,000K before nuclear fusion can start. |
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Term
| What is the role of rotation in the process of stellar birth? |
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Definition
| The rotation of a cloud of interstellar dust and gas helps contract the mass into a star. |
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Term
| What is the role of magnetism in the process of stellar birth? |
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Definition
| Manetism makes gas clouds contract, it heats up, and atomic encounters become violent enough to (partly) ionize the gases. |
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Term
| What Is the Role of Heat in the Process of Stellar Birth? |
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Definition
| For a star to be formed the prevailing minimum temperatures in the environment should be 10,000 degrees Celsius. Heat helps in the fusion and lowers gravitational pull of elements that a make up a stellar star. |
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Term
| What is the evolutionary track? |
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Definition
A graphical representation of a star's life as a path on the Hertzsprung-Ruseel diagram.
Moving through a track on the HR diagram means as the star ages it traces out a path on the HR diagram. The brightness and temperature and size change. The sequence is roughly as follows: Pre-mainsequence, main sequence (there is a little motion within the main sequence as the star ages) then on up to Red Giant, and then depending on mass onto the asymptotic giant branch, perhaps multiple visits there with interspersing red giant phases, then if the star becomes a variable (like a RR Lyrae star or a Cepheid for more massive progenitors) it will be in particular parts of the diagram. Finally, for a low enough mass star it becomes a hot small white dwarf in the lower left corner. |
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Term
| Why do stars tend to form in groups? |
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Definition
| This would be due to star-cluterring. A grouping of any where from a dozen to a million stars that formed at he same time from the same cloud of interstellar gas. Stars in clusters are useful to aid our understanding of stellar evolution because, within a given cluster, stars are all roughly the same age and chemical composition and lie at roughly the same distance from Earth. |
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Term
| Why does the evolution of a protostar slow down as it approaches the main sequence? |
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Definition
The cause of this slowdown is heat 14even gravity must struggle to compress a hot object.
The contraction is governed largely by the rate at which the protostar 19s internal energy can be radiated away into space. The greater this radiation of internal energy 14that is, the more rapidly energy moves through the star to escape from its surface 14the faster the contraction occurs. As the luminosity decreases, so too does the contraction rate. |
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Term
| In what ways do the formative stages of high-mass stars differ from those of stars like the Sun? |
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Definition
| These stars form so hot that they rapidly create a cavity in the nebulas where they formed, and put out large amounts of UV. They evolve rapidly onto and off of the Main Sequence, reaching the red giant stage in a time when the Sun was barely settling down. And of course they end their lives, going nova, or supernova, in a relatively short time. |
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Term
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Definition
| Remnant of a fragment of collapsing gas and dust that did not contain enough mass to initiate core nuclear fusion. Such objects are frozen somewhere along their pre-main-sequence contraction phase, continually cooling into compact dark objects. Because of their small sizes and low temperatures they are extremely difficult to detect observationally. |
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Term
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Definition
| Protostar in the late stages of formation, often exhibiting violent surface activity. T Tauri stars have been observed to brighten noticeably in a short period of time, consistent with the idea of rapid evolution during this final phase of stellar formation. |
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Term
| Stars live much longer than we do, so how do astronomers test the accuracy of theories of star formation? |
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Definition
| Simple...there are other stars of the same mass but have different ages to confirm the theory of star formation. The sun for example has solar analogs and/or twins. From that data alone, we have searched the local neighborhood and found different stars of the same mass to be at a different stage of evolution as the sun. The almost similarly-massed Alpha Centauri is now a bit brighter and based on stellar seismology of this star, it is a billion years older than the Sun. |
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Term
| Why must astronomers use radio and infrared radiation to study pre-stellar objects? |
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Definition
he reason why pre-stellar objects are not plotted on a HR Diagram is because these objects have not yet begun thermonuclear reactions at their core. In other words, they are not yet stars and so they are not plotted on an HR Diagram because these diagrams only show the distribution of stars - main sequence stars, giant and super giant stars, and white dwarfs.
Since pre-stellar objects have not yet begun thermonuclear reactions, they are not hot enough to give off any light and so you can not really observe them in visible light. However, these objects do give off infrared radiation (heat) and radio waves and so they can be observed if you use detectors that are sensitive to these forms of electromagnetic radiation. |
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Term
| Why has it been difficult until recently to demonstrate that stars and protostars actually exist within star-forming regions? |
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Definition
| I guess a variety of reasons, protostars are faint and shine mainly in the infrared, star forming regions contain a lot of obscuring gas and dust. |
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Term
| What is a shock wave? Of what significance are shock waves in star formation? |
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Definition
The smaller occurs when a star goes nova and sends out an energy wave that carries away lighter gasses and dust. This may reduce star formation by removing material that would have been drawn together during star formation.
The larger shock waves result when galaxies collide. As a result of the interacting massive centers of gravity, there are created differences in acceleration and momentum between the two combining masses. Some stars are orbiting their galactic center more in alignment with the other galactic center than other stars. This extra coincidence gives them a higher absolute speed than the others. The sheer line between areas moving together at different speeds produces a higher concentration of gasses and dust than would otherwise be the case. These areas come alive with millions or billions of new star formations. |
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Term
| Explain the usefulness of the H–R diagram in studying the evolution of stars. Why can’t evolutionary stages 1–3 be plotted on the diagram? |
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Definition
The easiest visible parameters of a star, are luminosity and temperature, and these represent the axes of the HR diagram. Over the life cycle of a star, these two parameters evolve and the result is that the star's evolution can be tracked on the HR diagram.
> Why can't evolutionary stages 1-3 be plotted on the diagram?
Not knowing what your text book calls "stages 1-3" I can only say that HR diagrams usually limit themselves to temperatures above 2,000 K or so. But the giant molecular clouds from which stars form are much colder than that, initially. Say 10K or so. |
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Term
| Compare the times necessary for the various stages in the formation of a star like the Sun. Why are some so short and others so long? |
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Definition
| Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. The times vary because some stars are more dense than others and some have different atmospheric pressures and temperatures applied to them. |
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Term
| What do star clusters and associations have to do with star formation? |
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Definition
| The end result of cloud collapse is a group of stars, all formed from the same parent cloud and lying in the same region of space. Such a collection of stars is called a star cluster. |
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Term
| Compare and contrast the properties of open clusters and globular clusters? |
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Definition
Open" in this case means that the stars in the cluster are not gravitationally bound, and will drift away from each other over time. Open clusters are the result of massive star forming in the spiral arms when a giant molecular cloud collapsed. If we see them, then they are still young and contain therefore many, hot white and blue stars.
Globular clusters are starclusters that ar bound by gravity, which makes them spherical and gives them their name. They are bigger than open clusters and are located in the halo of the galaxies. They contain old stars, as no starforming is going on anymore, so the young, massive blue stars have disappeared. Globular clusters are the oldest objects in the galaxy. |
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Term
| How can we tell if a star cluster is young or old? |
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Definition
| we see many large, hot stars in the cluster. B is the closest answer, but astronomers actually look at H-R diagrams of the cluster (which tell temperature and absolute magnitude). By matching these diagrams to what would be expected for a main sequence cluseter, we can date them by finding the location of the "turn off," or the point at which the diagram leaves the normal main sequence track. Since we know roughly the characteristics of the stars right at the the turn off, we can predict what their lifetimes should be based on stellar modeling. That age is then very close to the age of the cluster (assuming all of the stars were formed at the same time, which in a cluster is a good assumption). |
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